1. Field of the Invention
The present invention relates to a control system for an internal combustion engine and, particularly, to a control system that accurately controls an air-fuel ratio in a combustion chamber of the internal combustion engine having an exhaust gas recirculation mechanism.
2. Description of the Related Art
Japanese Patent Laid-open No. H08-61112 discloses a control system for a diesel engine having an exhaust gas recirculation mechanism. According to the disclosed control system, a gas amount Gf supplied to a combustion chamber is calculated using a map which is set according to a depressed amount of an accelerator and an engine rotational speed, and an intake air amount Ga is detected by an intake air amount sensor. Further, the intake air amount Ga is subtracted from the gas amount Gf to calculate a recirculated gas flow rate Ge. An air amount Gae in exhaust gases recirculated through the exhaust gas recirculation mechanism is calculated according to the recirculated gas amount Ge and an oxygen concentration OX in the exhaust gases detected by an oxygen concentration sensor provided in the exhaust system. An air amount Gcyl supplied to the combustion chamber is calculated as (Ga+Gae), and a fuel injection amount is calculated according to the air amount Gcyl.
With respect to the internal combustion engine, especially to the diesel engine, an emission amount of NOx or particulate matter (PM) has been strictly regulated in recent years. Accordingly, it is extremely difficult to obtain the required performance using the conventional control method which supplies an excessive amount of air compared to the fuel supply amount. Therefore, it is necessary to appropriately control the intake air amount of the engine and the air-fuel ratio of the air-fuel mixture in the combustion chamber.
In the conventional control system described above, a technique, wherein a target value of the intake air amount is set according to an engine operating condition and the actual intake air amount is controlled based on the target value, is not implemented. Consequently, according to the conventional control system, in recent years it has been difficult to perform the intake air amount control and the fuel supply control which satisfy a strict demand of the engine performance.